The present invention is directed to a process for making barium titanate and, more particularly, to a hydrothermal process for making barium titanate powders.
Barium titanate has a high dielectric constant, which makes it a useful material in the formation of a multilayer ceramic capacitor ("MLC"). It is widely used in a doped form as the dielectric material in MLC applications. MLC's comprise alternating layers of dielectric and electrical conductor materials. The dielectric layers of an MLC are usually prepared from a high solids dispersion, which typically comprises barium titanate powder and a polymeric binder in an aqueous or non-aqueous solvent. The dispersion, or slip, is cast or coated to provide a "green" layer of ceramic dielectric which are then coated with conductor materials in a pattern and are then stacked to provide a laminate of alternating layers of green ceramic dielectric and conductor. The stacks are diced into MLC-sized cubes which are heated to burn off organic materials, such as binder and dispersant, and are then fired to sinter the particles of barium titanate-based material to form a capacitor structure with laminated, dense ceramic dielectric and conductor layers. During sintering increased ceramic dielectric density is achieved as a result of the fusion and consolidation of the particles to form grains. Even with the use of grain growth inhibitors, ceramic grain size in an MLC dielectric layer is typically larger, for example by a factor of between 3 and 5, than the size of the original primary particles. Moreover, not all porosity is removed during the sintering process. Typically, 2 to 10% porosity remains in MLC dielectric layers. These pores, or hole defects, in the dielectric layer, tend to be larger in larger grain size ceramics. Certain critical capacitor properties, such as break down voltage and DC leakage, are influenced by dielectric thickness, grain size and pore defects. For instance, it is believed that effective dielectric layers need to be several, for example at least 3 to 5, grains thick. Because a defect in any one of the layers of an MLC can be fatal to its performance, MLC's are manufactured with a sufficient thickness of dielectric layer to effectively reduce the impact of ceramic defects which can be caused by random large grains or pores, adversely affect the properties of the MLC.
The miniaturization of electronic components and desire to increase the volumetric efficiency of capacitors has led to the production of dielectric layers of ever-decreasing thickness. As wet casting processes and suspension dispersions have been refined to provide thinner commercial dielectric layers, a smaller particle size dielectric is required with an equiaxed morphology for optimum packing. If the particle diameters approach the magnitude of the dielectric thickness, the voids inherent to the randomly packed layers would represent a large percentage of the total thickness, and may be sufficiently large to produce a short-circuit across the dielectric layer rendering the dielectric layer useless. Subsequently, MLC manufacturers typically use submicron (for example, less than 400 nm) spherical dielectric powders.
Barium titanate powders produced by conventional processes, for example calcination, oxalate-derived, and sol gel-derived methods, may require additional crystallization heat treatments, and/or milling prior to use in forming operations. Improper application of the respective secondary processes may produce an aggregated powder unsuited for use in ultra-thin layers and powder surfaces which exhibit a non-stoichiometric Ba/Ti due to the incongruent dissolution of barium. The large particles, and/or strongly-agglomerated fine particles, formed in these processes may have sizes substantially larger than 1 .mu.m. These particles and/or agglomerates are not readily amenable to the production of MLC's with fine grained, ultra-thin dielectric layers of less than between about 4-5 .mu.m.
Hydrothermal processes are known, for example, as disclosed by Menashi et al., in U.S. Pat. No. 4,832,939. Menashi et al. disclose a method of hydrothermally synthesizing stoichiometric, submicron, dispersible doped and undoped barium titanate and dielectric compositions of barium titanate which have very narrow particle size distributions. In one embodiment, barium titanate powder is produced by introducing a solution of 0.5 to 1.0 molar Ba(OH).sub.2 heated to a temperature between 70.degree. C.-90.degree. C., into a vigorously stirred slurry of a high surface area hydrous titania at a temperature ranging between 60.degree. C.-150.degree. C. at a constant rate over a time period of less than five minutes. The introduction of Ba(OH).sub.2 continues until the Ba/Ti mole ratio in the slurry is between 1.1 to 1.3. The slurry is then held at temperature for 10 to 30 minutes so that 95 to 98 percent of the TiO.sub.2, is converted to BaTiO.sub.3. The slurry is then heated to an elevated temperature, preferably at least 175.degree. C., to ensure complete conversion of the tetravalent hydrous oxide to a stoichiometric perovskite. After cooling to an appropriate temperature, the slurry is pressure filtered to give a cake of stoichiometric BaTiO.sub.3 containing 80 to 85 weight percent solids. The product is then washed with either water or a 0.01 to 0.02M Ba(OH).sub.2 solution. The wet cake is then dried resulting in a high purity, stoichiometric barium titanate powder having a primary particle size in the range between 0.05 and 0.4 micron with a very narrow particle size distribution.
Recent hydrothermal studies by Kumazawa et al., "Preparation of barium titanate ultrafine particles from amorphous titania by a hydrothermal method and specific dielectric constants of sintered discs of the prepared particles," Journal of Matl. Science, 1996, pp. 2599-2602 ("Kumazawa"), and Wada et al., "Preparation of Barium Titanate Fine Particles by Hydrothermal Method and Their Characterization," Journal of the Ceramic Society of Japan, 103 [12] 1220-1227 (1995) ("Wada"), employed simple non-injection systems, wherein suspensions of barium hydroxide octahydrate (Ba(OH).sub.2.8H.sub.2 O) and hydrated titanium oxide gel (TiO.sub.x (OH).sub.Y) were introduced into an autoclave at room temperature, sealed to the atmosphere, and ramped up to a final reaction temperature under agitation. In both studies, the particle size was reported to strongly correlate on the precursor Ba/Ti molar ratio. A decrease in the mean particle size was observed with an increase of the precursor Ba/Ti until a critical ratio, Ba/Ti.sub.crit, was reached after which the particle size remained constant with additional increases of the precursor Ba/Ti. The Ba/Ti.sub.crit that resulted from the Kumazawa hydrothermally-derived BaTiO.sub.3 powders was about 2.0, in contrast to a Ba/Ti.sub.crit of about 20.0 reported by Wada.